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In reality the influence of this bias in ground calibration is even more significant, because for segments 0 and 11, which cannot be calibrated directly (because of spread caused by the effect of direct light) the calibration constant was linearly extrapolated from segments 1 and 10 and the bias of 4 mm become 8 mm in segments 0 and 11.
The error mentioned above was the biggest, but not the only source of systematic errors in asymmetry calibration. When I compared the calibrations based on data collected in August 2008 and in October 2009, the difference shows systematic behaviour with position along the crystal:
(C&A 21 June please look at the following section second)
This effect looks smaller than effect of the error in ground calibration procedure, but it affects all crystals. The change of asymmetry slope demonstrate substantial variations from crystal to crystal, but strongly correlated with the time drift of energy scale
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